Fate and role of metastable pyroxenes in the subduction process

亚稳态辉石在俯冲过程中的命运和作用

• 批准号: 1722969
• 负责人: Przemyslaw Dera
• 金额: $ 36万
• 依托单位: University of Hawaii
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-07-01 至 2020-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1722969&HistoricalAwards=false
• 关键词: Fate; role; metastable; pyroxenes; subduction; Fate; role; metastable; pyroxenes; subduction

Subduction zones are geological environments that play a particularly important role in plate tectonics and recycling of crustal material into the mantle. A majority of earthquakes with magnitude greater than 5.5, and much of volcanic activity, both of which dramatically affect human civilization, are distributed along the boundaries of tectonic plates, and originate within subduction zones (e.g. the Ring of Fire). Pyroxene minerals are present within the subducting slabs in large quantities and their transformations are relevant for controlling the subduction process. Better understanding of the transformations these minerals undergo on microscopic scale will improve our ability to predict earthquakes and volcanic eruptions, and may help to mitigate some of the damage they cause. Within this project the team will use a diamond anvil cell apparatus, miniature hand-held devices capable of generating pressure and temperature conditions found at hundreds of kilometers of depth within the Earth interior, and will conduct an investigation of transformations that different members of the pyroxene family undergo, while transported into the Earth's mantle within the subducting slabs. The results of these experiments will be complemented by quantum mechanical calculations and will be incorporated into geophysical modeling software to assess the effects on the subduction process and mantle convection. Besides specific scientific goals, within this project the investigators will focus on educating the broader scientific community and the general public about mineral physics research through activities such as participation in live interviews on Hawaiian TV and participation in the CIDER program. While most mantle pyroxenes disappear below the transition zone (either dissolved into garnet, transformed to dense high-pressure polymorphs, or decomposed into oxides and dense silicates), at convergent margins the conditions of the cold subducting slab are characterized by much lower temperatures and promote metastable preservation of minerals such as pyroxenes and olivine beyond their thermodynamic stability fields, with possible consequences for slab buoyancy and earthquake triggering. The pyroxene (Px) structure is remarkably resilient, and if the temperature does not exceed 1000°C, can be easily preserved to pressures as high as 60 GPa (depths well beyond 1000 km). The general topology of the Px stable phase diagram has been well-established, however, until recently, very few experimental studies ventured into the metastable compression regime, most important for cold subduction zones. With previous NSF funding the PI and collaborators used synchrotron single-crystal micro-diffraction technique and discovered a number of intriguing, previously unknown phase transformations in the pyroxene family, occurring on compression at temperatures lower than the pyrolite geotherm. These transformations discontinuously change the thermodynamic properties of pyroxenes as a function of depth within the subducting slab, and are present across a wide range of pyroxene compositions. This current project continues exploration of the metastable phase boundaries and geodynamic consequences of the metastable polymorphism of pyroxenes. The team is aiming to explore three primary hypotheses: (H1) Even though the structural modifications introduced by the metastable phase transitions are subtle, and accompanying density discontinuities small, their consequences on thermodynamic and elastic properties, PT slopes of phase boundaries, as well as buoyancy relations within subduction zone can be significant; (H2) Defects, such as dislocations caused by metastable phase transitions, have significant consequences for pyroxene phase diagrams, and may leave quenchable signatures that will be recognizable in exhumed rock samples; (H3) Well-defined crystalline phases with penta-coordinated silicon are present as intermediates during densification of most Px compositions within cold slabs, and may significantly affect reactivity and rheology.

俯冲带是地质环境，在板块构造和将地壳材料回收到地幔中起着特别重要的作用。大多数地震大于5.5，大部分火山活动（都极大地影响人类文明，都沿构造板的边界分布，并源于俯冲带（例如火环）。辉石矿物质存在于大量俯冲板中，它们的转换与控制俯冲过程有关。更好地了解这些矿物在微观范围内经历的转变将提高我们预测地震和火山喷发的能力，并可能有助于减轻它们造成的某些损害。在该项目中，该团队将使用钻石砧型式设备，微型手持设备，能够产生压力和温度条件，在地球内部内数百公里的深度处发现，并将进行转换的投资，这些转换的投资，这些转换是在耐岩烯家族的不同成员中，同时将其运输到地球内的地面内。这些实验的结果将通过量子机械计算来完成，并将纳入地球物理建模软件中，以评估对俯冲过程和地幔结构的影响。除具体的科学目标外，在该项目中，调查人员将通过诸如参加夏威夷电视的现场访谈和参与苹果酒计划的活动，专注于更广泛的科学界和公众有关矿物质研究的教育。 While most mantle pyroxenes disappear below the transition zone (either dissolved into garnet, transformed to dense high-pressure polymorphs, or decomposed into oxides and dense silicones), at convergent margins the conditions of the cold subducting slab are characterised by much lower temperatures and promote metastable preservation of minerals such as pyroxenes and olivine beyond their thermodynamic stability fields, with平板浮力和地震触发的可能后果。辉石（PX）结构具有明显的弹性，如果温度不超过1000°C，则可以很容易地保留到高达60 GPA的压力（深度远远超过1000 km）。然而，直到最近，很少有实验研究冒险进入亚稳态压缩方案，这对于冷俯冲带最重要，因此PX稳定相图的一般拓扑已经良好。借助以前的NSF资金，PI和合作者使用了同步子单晶微伸缩技术，并发现了辉石家族中许多有趣的，以前未知的相位转换，该技术在低于Pyrolite Geotherm的温度下发生在压缩上。这些转换不连续地改变了辉石的热力学特性，这是俯冲平板内深度的函数，并且存在于广泛的辉石组成范围内。当前的项目继续探索焦氧烯亚稳态多态性的亚稳相边界和地球动力学后果。该团队的目的是探索三个主要假设：（H1）即使可稳态相变引入的结构修改是微妙的，并且参与密度不连续性很小，但它们对热力学和弹性的后果，对热力学和弹性的后果，PT的PT插槽，以及在俯冲区域内的生产范围以及俯冲区域内的生产力； （H2）缺陷，例如由亚稳态过渡引起的位错，对辉石相图产生了重大影响，并且可能会留下可淬灭的签名，这些签名将在被灌肠的岩石样品中识别出来； （H3）在冷板内大多数PX组成的撤离过程中，具有五角形配位硅的明确定义的结晶相存在为中间体，并且可能会显着影响反应性和流变学。